Lately, the IPv6 service seems pretty reliable with my system. That was a problem for some time. It may have been a bad interaction between the Comcast plant and my local routers, but it just went away. It may have been a Comcast reconfiguration that helped us, but it also could have been an Asus firmware update. I am now running ASUSWRT-MERLIN firmware on the Asus RT-N66U router, which I can highly recommend as an expanded and improved version of the Asus distribution.(It cured a long-standing issue with JFFS2 overflow.)
Again, I highlight the DSL Reports speed test, which checks your "real world" network performance, including the dread "buffer bloat". Today is Boxing Day (Dec. 26), and DSL-Reports is giving me a so-so report and a good report. Here they are, separated by half an hour:
For comparison, between these two tests, Comcast's own speed test shows this:
This is the available speed within the Comcast network, which seems to be the best possible result -- not fully representative of what you experience with a random Internet connection -- even if the server is fast and well-connected. (Note that the driving distance from Branford CT to Boston is really about 144 miles, not under 50. Go figure.)